Of color display devices used as, for example, computers and televisions for image display, a display device using a plasma display panel (hereinafter referred to as “PDP”) has received attention in recent years as a large-size, thin and lightweight color display device.
A plasma display device using this PDP produces a display in full color by additive mixture of so-called three primary colors (red, green and blue). For display in full color, the plasma display device is provided with phosphor layers which emit respective lights having these respective three primary colors, that is, red (R), green (G) and blue (B). Phosphor particles of each phosphor layer are excited by ultraviolet rays generated in a discharge cell of the PDP, thereby producing visible colored light.
Known compounds used for the above-described phosphors having the respective colors include (YGd)BO3:Eu3+ which emits red light, Zn2SiO4:Mn2+ which emits green light and BaMgAl10O17:Eu2+ which emits blue light. Each of these phosphors is made by mixing specified raw materials and then firing a resulting mixture at a high temperature of at least 1,000° C. for solid-phase reaction (refer to, for example, “Phosphor Handbook” published by Ohmsha, Ltd., pp. 219 and 220). Phosphor particles obtained by this firing are ground and sifted before use (so that red and green phosphor particles have a mean particle diameter of 2 μm to 5 μm, while blue phosphor particles have a mean particle diameter of 3 μm to 10 μm). The phosphor particles are ground and sifted (classified) for the following reason. In general, paste including the phosphor particles having each color is applied by screen printing to form a phosphor layer in the PDP. A surface of the paste applied easily becomes smoother if the particle diameters of the phosphor are smaller and more uniform (in particle size distribution). In other words, the smaller and more uniform the particle diameters of the phosphor, and the more spherical the phosphor particles, the smoother the applied surface. Accordingly, packing density and a light-emitting surface area of the phosphor particles in the phosphor layer conceivably increase, thus increasing luminance of the plasma display device.
However, reducing the particle diameters of the phosphor particles increases a surface area of the phosphor and thus increases a number of defects in the phosphor. For this reason, a large amount of water, carbon dioxide or hydrocarbon-containing organic substances easily adheres to a surface of the phosphor. For a blue phosphor, including a divalent Eu ion as a luminescence center, such as Ba1−xMgAl10O17:Eux or Ba1−x−ySryMgAl10O17:Eux, in particular, its crystal structure has a layer structure (refer to, for example, “Display and Imaging”, 1999, Vol. 7, pp. 225–234). This layer structure includes an oxygen (O) vacancy in the vicinity of a layer (Ba—O layer) including a Ba atom, and the smaller the particle diameters, the more these vacancies problematically increase in number (refer to, for example, “OYO BUTSURI (Applied Physics)”, Vol. 70, No. 3, 2001, p. 310). FIG. 6 schematically illustrates a structure of a Ba—O layer of the blue phosphor, Ba1−XMgAl10O17:EuX.
For the above reason, water existing in air or hydrocarbon-containing gas selectively adsorbs on a surface of such a Ba—O layer. Such water and gas are released into the panel in large amounts in a panel manufacturing process and react with the phosphor and MgO during discharge, thus problematically degrading luminance, causing a change in chromaticity (which leads to a color shift or a burn on a screen), and reducing a drive margin and raising discharge voltage.
It is also known that the defects increase further in number as vacuum ultraviolet rays (VUV), having a wavelength of 147 nm, and which are generated by discharge in driving the panel, are absorbed, thus problematically degrading luminance of the phosphor further (refer to, for example, “Technical Report” published by the Institute of Electronics, Information and Communication Engineers, EID99–94, Jan. 27, 2000).
To solve these problems, the following conventional method (such as disclosed in Japanese Patent Unexamined Publication No. 2001-55567) is devised. According to this method, an entire surface of a phosphor is covered with a crystal coating of Al2O3 for a purpose of repairing a vacancy in the vicinity of the Ba—O layer.
However, covering the entire surface of the phosphor with the crystal coating of Al2O3 causes ultraviolet rays to be absorbed, thus problematically reducing emission luminance of the phosphor.
In view of the problems discussed above, the present invention aims to suppress degradation of luminance of a phosphor or a change in chromaticity, or improve discharge characteristics by suppressing adsorption of water on the surface of the blue phosphor.